Medical School · Year 2 · Immunology · includes a quiz and discussion video

Lecture 1: Innate Immunity

Unit 2.7: Immunology

Learning Objectives

By the end of this lecture, students will be able to:

1. Describe the components and functions of the innate immune system
2. Explain pattern recognition receptors and pathogen-associated molecular patterns
3. Describe the cells of innate immunity and their roles
4. Explain the complement system and its activation pathways
5. Describe the inflammatory response and acute phase reaction
6. Explain the interface between innate and adaptive immunity

Lecture Outline

I. Overview of Innate Immunity

The innate immune system represents the body's first line of defense against pathogens, providing immediate protection through mechanisms that are evolutionarily ancient and highly conserved across species. Unlike the adaptive immune system, innate immunity responds within minutes to hours of encountering a pathogen, requiring no prior exposure or learning period. The fundamental characteristic of innate immunity is its reliance on broad pattern recognition rather than specific antigen recognition, enabling it to respond to diverse pathogens using a limited repertoire of germline-encoded receptors.

The innate immune system discriminates between self and non-self by recognizing pathogen-associated molecular patterns (PAMPs), which are molecular structures common to groups of pathogens but absent from host cells. Examples include lipopolysaccharide found exclusively on gram-negative bacteria and double-stranded RNA characteristic of certain viruses. This recognition system allows the innate immune response to mount defenses against previously unencountered pathogens immediately upon first exposure, without requiring the clonal expansion that characterizes adaptive immunity.

Importantly, while innate immunity lacks the immunological memory that defines adaptive responses, it serves several critical functions beyond direct pathogen elimination. The innate immune system initiates and directs the adaptive response by processing and presenting antigens, providing costimulatory signals, and secreting cytokines that shape the type of adaptive response generated. Dendritic cells exemplify this bridging function, surveying tissues for pathogens and carrying antigen information to lymph nodes where they activate naive T cells.

The components of innate immunity can be organized into three categories: physical and chemical barriers that prevent pathogen entry, cellular defenses including phagocytes and natural killer cells, and soluble factors such as complement proteins and cytokines. Together, these elements form an integrated defense system capable of containing most infections without adaptive immune involvement, while simultaneously alerting and preparing the adaptive system for pathogens that evade these initial defenses.

<image> Panel A: Diagram illustrating the timeline comparison between innate and adaptive immune responses, showing innate immunity responding within 0-12 hours while adaptive immunity requires 1-2 weeks for primary response, with memory response occurring within days Panel B: Schematic of the three main components of innate immunity displayed as concentric layers - physical barriers (skin, mucosa) as outer layer, cellular components (neutrophils, macrophages, dendritic cells, NK cells) as middle layer, and soluble factors (complement, cytokines, acute phase proteins) as inner layer Panel C: Comparison table visualization showing key differences between innate and adaptive immunity including speed (fast vs slow), specificity (broad patterns vs specific epitopes), memory (absent vs present), and receptor diversity (germline-encoded vs somatically rearranged) Panel D: Flowchart demonstrating the sequential events in innate immunity from pathogen entry through barrier breach, PAMP recognition, inflammatory response, phagocytosis, and ultimately initiation of adaptive immunity </image>

II. Physical and Chemical Barriers

The skin serves as the body's most extensive physical barrier, presenting a formidable challenge to potential pathogens through its multilayered structure and continuous renewal. The outermost stratum corneum consists of tightly packed, keratinized dead cells that form a waterproof, impenetrable surface under normal conditions. This physical barrier is enhanced by the skin's relatively dry, acidic environment (pH 5.5) created by fatty acids in sebum, which inhibits bacterial and fungal growth. The skin also harbors a resident microbiota that occupies ecological niches and produces antimicrobial substances, providing competitive exclusion against pathogenic colonization.

Mucosal surfaces lining the respiratory, gastrointestinal, and urogenital tracts present a different challenge, as these surfaces must remain permeable for their physiological functions while still preventing pathogen entry. The mucus layer overlying these epithelia serves as a gel-like trap that immobilizes pathogens and facilitates their expulsion through mechanical clearance mechanisms. In the respiratory tract, the mucociliary escalator coordinates ciliary beating to propel mucus and trapped particles upward toward the pharynx, where they are swallowed and destroyed in the acidic stomach environment.

Chemical defenses complement physical barriers throughout the body. Lysozyme, present in tears, saliva, and respiratory secretions, enzymatically degrades the peptidoglycan cell wall of gram-positive bacteria. Lactoferrin sequesters iron, an essential nutrient for bacterial growth, while phospholipase A2 destroys bacterial membranes. In the stomach, hydrochloric acid creates an extremely acidic environment (pH 1-2) that kills most ingested microorganisms. The small intestine relies on bile acids and pancreatic enzymes for antimicrobial activity, while the large intestine depends heavily on competitive exclusion by the resident microbiota.

Antimicrobial peptides (AMPs) represent a particularly important category of chemical defenses found throughout barrier tissues. Defensins, produced by epithelial cells and neutrophils, insert into bacterial membranes and form pores that disrupt membrane integrity. Human defensins are classified as alpha-defensins (found in neutrophil granules and Paneth cells) and beta-defensins (expressed by epithelial cells throughout the body). Cathelicidin (LL-37), another important AMP, not only kills bacteria directly but also serves as a chemoattractant for immune cells and promotes wound healing. These peptides provide broad-spectrum antimicrobial activity against bacteria, fungi, and enveloped viruses.

<image> Panel A: Cross-sectional diagram of skin showing the epidermis layers from stratum basale to stratum corneum, with annotations highlighting keratinization, antimicrobial peptide secretion from keratinocytes, Langerhans cells as resident dendritic cells, and the acidic pH of the skin surface Panel B: Illustration of the mucociliary escalator in respiratory epithelium showing ciliated cells with coordinated beating pattern, goblet cells secreting mucus, and trapped bacteria being transported upward, with inset showing mucus composition including lysozyme and secretory IgA Panel C: Chemical structure representations of antimicrobial peptides showing alpha-defensin and beta-defensin structures, with mechanism illustration of membrane pore formation in a bacterial cell wall Panel D: Summary diagram of chemical barriers at different body sites showing stomach acid, vaginal pH, lysozyme in tears/saliva, bile acids in small intestine, and commensal bacteria in large intestine with their antimicrobial functions </image>

III. Pattern Recognition Receptors

Pattern recognition receptors (PRRs) are germline-encoded sensors that detect conserved molecular structures unique to microorganisms, enabling the innate immune system to respond rapidly to infection without prior antigen exposure. These receptors recognize two categories of molecular patterns: pathogen-associated molecular patterns (PAMPs) found on microorganisms but absent from host cells, and damage-associated molecular patterns (DAMPs) released from damaged or dying host cells. This dual recognition system allows the innate immune response to detect both infectious agents and sterile tissue injury.

Toll-like receptors (TLRs) comprise the best-characterized family of PRRs, with ten functional members identified in humans. TLRs are type I transmembrane proteins containing extracellular leucine-rich repeat domains that recognize PAMPs and intracellular Toll/IL-1 receptor (TIR) domains that initiate signaling. The localization of TLRs reflects their ligand specificity: cell surface TLRs (TLR1, TLR2, TLR4, TLR5, TLR6) detect microbial membrane components like lipopolysaccharide (TLR4) and flagellin (TLR5), while endosomal TLRs (TLR3, TLR7, TLR8, TLR9) detect nucleic acids that become accessible only after microorganisms are internalized and degraded.

TLR signaling proceeds through two main pathways depending on the adaptor proteins recruited. The MyD88-dependent pathway, utilized by all TLRs except TLR3, leads to activation of NF-κB and production of pro-inflammatory cytokines including TNF-α, IL-1β, and IL-6. The TRIF-dependent pathway, used by TLR3 and TLR4, induces type I interferon production essential for antiviral responses. TLR4, uniquely among TLRs, can signal through both pathways, explaining the potent inflammatory response triggered by lipopolysaccharide during gram-negative bacterial infections.

Beyond TLRs, several other PRR families contribute to innate immune detection. NOD-like receptors (NLRs) are cytoplasmic sensors that detect bacterial components like peptidoglycan fragments and cellular stress signals. NOD1 and NOD2 recognize distinct portions of peptidoglycan, while other NLRs like NLRP3 form inflammasomes that activate caspase-1 and promote IL-1β and IL-18 secretion. RIG-I-like receptors (RLRs) detect viral RNA in the cytoplasm, triggering type I interferon production. C-type lectin receptors (CLRs) on cell surfaces recognize carbohydrate structures, with Dectin-1 detecting fungal β-glucans being particularly important for antifungal immunity.

<image> Panel A: Illustration of TLR family members showing their cellular localization - TLR1, TLR2, TLR4, TLR5, and TLR6 on the plasma membrane, and TLR3, TLR7, TLR8, and TLR9 in the endosomal membrane, with their respective PAMP ligands (LPS, flagellin, lipoteichoic acid, dsRNA, ssRNA, CpG DNA) indicated Panel B: Detailed signaling diagram showing MyD88-dependent pathway leading to NF-κB activation and pro-inflammatory cytokine production, and TRIF-dependent pathway leading to IRF3/7 activation and type I interferon production, with TLR4 engaging both pathways Panel C: Structural comparison of different PRR families showing TLRs (leucine-rich repeats), NLRs (NOD domains and leucine-rich repeats), RLRs (helicase domains), and CLRs (carbohydrate recognition domains) with their typical ligands Panel D: NLRP3 inflammasome assembly diagram showing NLRP3 sensor activation by various DAMPs and PAMPs, ASC adaptor recruitment, caspase-1 activation, and subsequent IL-1β and IL-18 processing and pyroptotic cell death </image>

IV. Cells of Innate Immunity - Phagocytes

Neutrophils are the most abundant circulating leukocytes, comprising 60-70% of white blood cells, and serve as the primary first responders to bacterial and fungal infections. These short-lived cells (half-life of approximately 6-8 hours in circulation) are produced in the bone marrow at remarkable rates, with approximately 10^11 neutrophils generated daily. Upon infection, bone marrow release accelerates, chemokine gradients guide neutrophil migration to inflammatory sites, and these cells rapidly accumulate to form the characteristic purulent exudate of bacterial infection. Neutrophils employ multiple killing mechanisms including phagocytosis, degranulation of antimicrobial proteins, generation of reactive oxygen species through the respiratory burst, and formation of neutrophil extracellular traps (NETs).

The respiratory burst in neutrophils represents a critical antimicrobial mechanism involving the NADPH oxidase complex, which generates superoxide anions that are subsequently converted to hydrogen peroxide and hypochlorous acid (bleach). This potent oxidative machinery is assembled on the phagosomal membrane, concentrating reactive oxygen species within the phagolysosome where ingested microorganisms are trapped. Deficiency of NADPH oxidase components causes chronic granulomatous disease (CGD), characterized by recurrent life-threatening infections with catalase-positive organisms that can neutralize their own hydrogen peroxide production. NETs represent a more recently discovered neutrophil defense mechanism involving the extrusion of chromatin decorated with antimicrobial proteins to trap and kill extracellular pathogens.

Macrophages are tissue-resident phagocytes derived from two sources: circulating monocytes that differentiate upon tissue entry, and embryonically-derived populations that self-renew throughout life. Different tissues harbor specialized macrophage populations with tissue-specific names and functions, including Kupffer cells in the liver, alveolar macrophages in the lungs, microglia in the brain, and osteoclasts in bone. Unlike neutrophils, macrophages are long-lived cells capable of sustained phagocytic activity and serve critical roles in tissue homeostasis, wound healing, and inflammation resolution in addition to antimicrobial defense.

Macrophage activation exists along a spectrum between two polarization states that reflect their functional plasticity. Classical activation (M1 polarization) is induced by interferon-γ and microbial products, enhancing bactericidal activity through increased reactive oxygen and nitrogen species production. Alternative activation (M2 polarization), induced by IL-4 and IL-13, promotes tissue repair, fibrosis, and anti-inflammatory functions. This plasticity allows macrophages to adopt different functional states depending on the tissue microenvironment, transitioning from pro-inflammatory roles during active infection to tissue repair roles during resolution. Macrophages also serve as professional antigen-presenting cells, processing ingested microorganisms and presenting peptide fragments on MHC class II molecules to activate CD4+ T cells.

<image> Panel A: Sequential illustration of neutrophil recruitment showing rolling on activated endothelium (selectin-mediated), firm adhesion (integrin-mediated), and transmigration through the vessel wall, with chemokine gradient guiding movement toward the infection site Panel B: Diagram of the NADPH oxidase complex showing its membrane-associated (gp91phox, p22phox) and cytosolic (p47phox, p67phox, Rac) components, with the electron transfer chain generating superoxide that is converted to hypochlorous acid by myeloperoxidase Panel C: Comparison of macrophage polarization states showing M1 macrophages (IFN-γ activated, pro-inflammatory, microbicidal) versus M2 macrophages (IL-4/IL-13 activated, tissue repair, anti-inflammatory), with key surface markers and secreted factors for each state Panel D: Phagocytosis mechanism showing pathogen recognition by opsonins (antibody, C3b), engulfment into phagosome, phagolysosome fusion, and killing by ROS, reactive nitrogen species, and granule enzymes, with bacteria being degraded and antigens processed for presentation </image>

V. Cells of Innate Immunity - Other Cells

Dendritic cells (DCs) are specialized antigen-presenting cells that serve as the critical link between innate and adaptive immunity. These cells exist in two functional states: immature DCs residing in peripheral tissues where they continuously sample the environment through macropinocytosis and receptor-mediated endocytosis, and mature DCs that have encountered antigen and migrated to secondary lymphoid organs to activate naive T cells. The maturation process involves dramatic phenotypic changes including upregulation of MHC molecules and costimulatory molecules (CD80, CD86, CD40), downregulation of phagocytic receptors, increased expression of CCR7 that directs migration to lymph nodes, and secretion of cytokines that shape the subsequent T cell response.

Dendritic cell subsets differ in their developmental origin, tissue distribution, and functional specialization. Conventional dendritic cells (cDCs) are the primary activators of naive T cells and comprise two major subsets: cDC1 cells, which excel at cross-presentation to CD8+ T cells and are critical for antiviral and antitumor immunity, and cDC2 cells, which primarily activate CD4+ T cells and are important for responses against extracellular pathogens. Plasmacytoid dendritic cells (pDCs) constitute a distinct population specialized for type I interferon production in response to viral infections, producing up to 1000 times more interferon-α than other cell types. This interferon-producing capacity makes pDCs essential for early antiviral responses.

Natural killer (NK) cells are innate lymphoid cells capable of killing virus-infected and tumor cells without prior sensitization. Unlike T cells, NK cells do not require MHC-restricted antigen recognition for activation. Instead, NK cell function is controlled by the balance between activating and inhibitory receptor signals. Inhibitory receptors, including killer immunoglobulin-like receptors (KIRs) in humans, recognize self-MHC class I molecules and prevent NK cell activation against healthy host cells. When target cells downregulate MHC class I (a common viral evasion strategy), inhibitory signaling is reduced and NK cells become activated—a concept termed "missing self" recognition.

NK cell activating receptors detect stress-induced ligands upregulated on infected or transformed cells. NKG2D recognizes MICA and MICB, stress-inducible molecules expressed on infected, transformed, or heat-shocked cells. Natural cytotoxicity receptors (NCRs) including NKp46, NKp44, and NKp30 detect various ligands on tumor cells and pathogen-infected cells. The Fc receptor CD16 (FcγRIIIa) enables antibody-dependent cellular cytotoxicity (ADCC), allowing NK cells to kill antibody-coated target cells. Upon activation, NK cells kill targets through perforin and granzyme release (similar to cytotoxic T cells) and produce interferon-γ to activate macrophages and shape adaptive immune responses.

<image> Panel A: Dendritic cell maturation journey showing immature DC in peripheral tissue capturing antigen, migration through lymphatics with progressive maturation, and arrival in lymph node T cell zone as mature DC with upregulated MHC class II and costimulatory molecules presenting antigen to naive T cell Panel B: Diagram comparing dendritic cell subsets - cDC1 (CD141+, XCR1+, cross-presentation), cDC2 (CD1c+, CD4+ T cell activation), and plasmacytoid DC (CD123+, type I IFN production), with their key surface markers and primary functions Panel C: NK cell activation model showing the balance between inhibitory receptor signals (KIRs binding MHC class I) and activating receptor signals (NKG2D binding MICA/MICB, NCRs binding stress ligands), demonstrating how missing self or induced self leads to target cell killing Panel D: NK cell effector mechanisms including directed granule release with perforin forming pores allowing granzyme B entry to induce apoptosis, death receptor pathway via FasL, IFN-γ secretion for macrophage activation, and ADCC through CD16 binding IgG Fc on target cells </image>

VI. The Complement System

The complement system comprises more than 30 plasma proteins that circulate in inactive forms and become sequentially activated through proteolytic cascades, ultimately producing effector molecules that eliminate pathogens through opsonization, direct lysis, and inflammation. Named for its ability to "complement" antibody-mediated killing, this system represents one of the oldest components of innate immunity, with homologs identified in invertebrates lacking adaptive immunity. The liver produces most complement proteins constitutively, maintaining serum concentrations that enable rapid responses to infection.

Three distinct pathways can initiate complement activation, all converging on the central component C3. The classical pathway is triggered when the C1 complex (comprising C1q, C1r, and C1s) binds to antibody-antigen complexes, with IgM pentamers and IgG molecules being the most efficient activators. Upon binding, C1s cleaves C4 and C2 to form the C3 convertase (C4b2a) on the target surface. The lectin pathway similarly generates C4b2a but is initiated by mannose-binding lectin (MBL) or ficolins recognizing carbohydrate patterns on pathogen surfaces, which activate MBL-associated serine proteases (MASPs) to cleave C4 and C2.

The alternative pathway operates continuously at low levels through spontaneous hydrolysis of C3 (C3 tickover), generating C3b that can bind to any nearby surface. On host cells, regulatory proteins rapidly inactivate deposited C3b, but pathogen surfaces lacking these regulators allow C3b to associate with factor B, which is then cleaved by factor D to form the alternative pathway C3 convertase (C3bBb). This convertase is stabilized by properdin and cleaves additional C3, generating an amplification loop that can massively increase C3b deposition. Importantly, the alternative pathway also amplifies complement activation initiated through the classical or lectin pathways.

All three pathways converge at C3 cleavage, generating C3b for opsonization and C3a as an anaphylatoxin. C3b binding to existing C3 convertases creates C5 convertases that cleave C5 into C5a (a potent anaphylatoxin and chemoattractant) and C5b. C5b initiates assembly of the membrane attack complex (MAC) by sequentially recruiting C6, C7, C8, and multiple C9 molecules, which polymerize to form transmembrane pores. These pores disrupt osmotic balance, causing cell lysis—particularly effective against gram-negative bacteria and some enveloped viruses. However, nucleated cells can often endocytose and eliminate MACs, making opsonization and inflammation the more significant complement functions against many pathogens.

<image> Panel A: Overview diagram of the three complement activation pathways - classical (antibody-antigen complexes activating C1), lectin (MBL/ficolins binding mannose), and alternative (spontaneous C3 hydrolysis) - all converging on C3 cleavage and then proceeding to the terminal pathway Panel B: Detailed illustration of C3 convertase formation and C3 cleavage, showing generation of C3b (opsonin that deposits on pathogen surface) and C3a (anaphylatoxin released into fluid phase), with the amplification loop of the alternative pathway highlighted Panel C: Membrane attack complex assembly showing sequential binding of C5b, C6, C7, C8, and polymerization of C9 to form a transmembrane pore in the bacterial outer membrane, with resulting osmotic lysis Panel D: Summary of complement effector functions showing opsonization (C3b coating enhancing phagocytosis via CR1/CR3), inflammation (C3a/C5a causing mast cell degranulation, vasodilation, and neutrophil chemotaxis), and direct lysis (MAC pores) </image>

VII. Complement Regulation and Deficiencies

Tight regulation of complement activation is essential to prevent uncontrolled inflammation and damage to host tissues. Multiple regulatory proteins operate at different points in the cascade, either accelerating the decay of convertases, acting as cofactors for factor I-mediated cleavage of C3b and C4b, or blocking MAC assembly. This redundant regulation reflects the potential danger posed by inappropriate complement activation and the evolutionary pressure to prevent self-damage while maintaining antimicrobial activity.

Fluid-phase regulators limit complement activation in plasma away from pathogen surfaces. C1 inhibitor (C1-INH) is a serine protease inhibitor that irreversibly inactivates C1r, C1s, and MASPs, preventing excessive classical and lectin pathway activation. Factor H is a key regulator of the alternative pathway, acting as a cofactor for factor I-mediated cleavage of C3b and accelerating decay of the alternative pathway C3 convertase. Factor H preferentially binds C3b deposited on host cells, which display sialic acid residues that enhance factor H binding, while pathogen surfaces lacking sialic acid are less protected.

Membrane-bound regulators protect host cells from complement-mediated attack. Decay-accelerating factor (DAF/CD55) accelerates the dissociation of C3 and C5 convertases on host cell surfaces. Membrane cofactor protein (MCP/CD46) serves as a cofactor for factor I-mediated cleavage of C3b and C4b. CD59 (protectin) prevents MAC formation by blocking C9 polymerization. The importance of these regulators is illustrated by paroxysmal nocturnal hemoglobinuria (PNH), in which a somatic mutation prevents synthesis of the glycosylphosphatidylinositol (GPI) anchor required for membrane attachment of DAF and CD59, leaving red blood cells vulnerable to complement-mediated lysis.

Complement deficiencies produce distinct clinical syndromes depending on which components are affected. Deficiencies of classical pathway components (C1q, C1r/s, C2, C4) are strongly associated with systemic lupus erythematosus, likely because complement normally facilitates clearance of apoptotic cells and immune complexes. C3 deficiency, though rare, causes severe recurrent pyogenic infections because all pathways converge on C3 and opsonization is profoundly impaired. Deficiencies of terminal pathway components (C5-C9) produce susceptibility specifically to Neisseria infections (meningococcal and gonococcal disease), with affected individuals having up to 1000-fold increased risk. Hereditary angioedema results from C1-INH deficiency, causing recurrent episodes of potentially life-threatening swelling of subcutaneous and submucosal tissues due to uncontrolled contact system activation and bradykinin generation.

<image> Panel A: Diagram showing regulatory proteins acting at different cascade stages - C1 inhibitor blocking C1r/s and MASPs, Factor H and Factor I cleaving C3b, DAF accelerating convertase decay, MCP as Factor I cofactor, and CD59 preventing C9 polymerization in MAC Panel B: Illustration of how Factor H distinguishes host from pathogen surfaces, showing Factor H binding to sialic acid residues on host cells and promoting C3b inactivation, while pathogen surfaces lacking sialic acid allow unchecked C3b amplification Panel C: Pathophysiology of paroxysmal nocturnal hemoglobinuria showing GPI anchor synthesis defect, absence of DAF and CD59 from red blood cell membranes, uncontrolled complement activation leading to intravascular hemolysis, and clinical manifestations including hemoglobinuria Panel D: Clinical manifestations of complement deficiencies arranged by pathway component - classical pathway defects (C1q, C2, C4) associated with SLE-like disease, C3 deficiency with severe pyogenic infections, terminal pathway defects (C5-C9) with Neisseria infections, and C1-INH deficiency with hereditary angioedema </image>

VIII. Inflammatory Response

Inflammation represents the tissue response to infection or injury, orchestrated by immune cells, vascular changes, and molecular mediators to contain pathogens, remove damaged tissue, and initiate repair. The cardinal signs of inflammation—rubor (redness), calor (heat), tumor (swelling), dolor (pain), and functio laesa (loss of function)—reflect the underlying vascular and cellular events that increase blood flow, enhance permeability, recruit leukocytes, and activate pain receptors. While essential for host defense, inflammation must be tightly controlled to prevent collateral tissue damage.

Vascular changes constitute the initial phase of inflammation, beginning within seconds of tissue injury or infection recognition. Vasodilation of arterioles and capillaries increases local blood flow, producing the redness and heat characteristic of inflammation. This vasodilation results from histamine release by mast cells, nitric oxide production by endothelium, and prostaglandin synthesis. Simultaneously, increased vascular permeability allows plasma proteins, including complement components and antibodies, to exit blood vessels and enter tissues. Permeability changes result from endothelial cell contraction creating intercellular gaps, mediated by histamine, leukotrienes, and bradykinin.

Leukocyte recruitment follows a well-characterized multistep process involving initial capture, rolling, activation, firm adhesion, and transmigration. Selectins (E-selectin and P-selectin) expressed on activated endothelium bind carbohydrate ligands on leukocytes, creating weak interactions that cause rolling along the vessel wall. This rolling slows leukocytes enough to encounter chemokines displayed on the endothelial surface. Chemokine receptor engagement activates integrins on leukocyte surfaces, increasing their affinity for immunoglobulin superfamily members (ICAM-1, VCAM-1) on endothelium and causing firm adhesion. Finally, leukocytes squeeze between endothelial cells (paracellular transmigration) or pass through them (transcellular transmigration), following chemokine gradients to the site of infection.

Pro-inflammatory cytokines, particularly TNF-α, IL-1β, and IL-6, coordinate both local and systemic inflammatory responses. These cytokines are produced primarily by activated macrophages and dendritic cells in response to PRR signaling. Locally, they induce endothelial activation, enhance vascular permeability, and promote leukocyte recruitment. Systemically, they act on the hypothalamus to induce fever, stimulate hepatocyte production of acute phase proteins, and promote neutrophil mobilization from bone marrow. The same cytokines can cause pathological inflammation when produced in excess, as occurs in septic shock where massive cytokine release leads to systemic vasodilation, hypotension, and multi-organ failure.

<image> Panel A: Cross-sectional diagram of a blood vessel showing vascular changes during inflammation - arteriolar dilation increasing blood flow, increased permeability with plasma protein leakage into tissues, and resulting edema formation, with histamine, prostaglandins, and bradykinin indicated as mediators Panel B: Multi-step leukocyte recruitment cascade showing rolling (selectin-mediated), chemokine activation, firm adhesion (integrin-mediated), and transmigration, with molecular participants (E-selectin, ICAM-1, LFA-1, chemokines) labeled at each step Panel C: Diagram of pro-inflammatory cytokine actions showing TNF-α, IL-1β, and IL-6 released from activated macrophages and their effects: endothelial activation, fever induction, acute phase protein synthesis, and bone marrow stimulation Panel D: Timeline illustration of inflammatory response showing immediate vascular phase (0-30 minutes), early cellular phase (4-6 hours, neutrophil predominant), and late cellular phase (24-48 hours, macrophage and lymphocyte predominant), with resolution mechanisms including lipoxins and resolvins </image>

IX. Acute Phase Response

The acute phase response represents the systemic manifestation of inflammation, involving coordinate changes in physiology and metabolism that support host defense against infection. Triggered by pro-inflammatory cytokines (primarily IL-6, IL-1β, and TNF-α) entering the circulation, this response produces fever, leukocytosis, and dramatic alterations in hepatic protein synthesis. While beneficial for combating infection, the acute phase response also produces the malaise, fatigue, and anorexia that characterize the sickness behavior accompanying infection.

Fever results from cytokine action on the hypothalamic thermoregulatory center. IL-1β, IL-6, and TNF-α either cross the blood-brain barrier or signal through endothelial cells to induce cyclooxygenase-2 expression and prostaglandin E2 (PGE2) synthesis in the hypothalamus. PGE2 acts on thermoregulatory neurons to raise the temperature set point, triggering heat conservation (peripheral vasoconstriction) and heat generation (shivering) until body temperature reaches the new elevated set point. Fever provides survival benefits by inhibiting growth of some pathogens, enhancing leukocyte mobility and function, and accelerating lymphocyte proliferation and antibody production. However, extreme fever (hyperpyrexia) can cause protein denaturation and organ damage.

Acute phase proteins undergo dramatic concentration changes during inflammation, with positive acute phase proteins increasing and negative acute phase proteins decreasing. C-reactive protein (CRP), the prototypical positive acute phase protein, can increase over 1000-fold within 24-48 hours. CRP binds phosphocholine on bacterial surfaces and apoptotic cells, activating the classical complement pathway and promoting phagocytosis. Other positive acute phase proteins include serum amyloid A (precursor of amyloid deposits in chronic inflammation), fibrinogen (increasing ESR), ferritin (iron sequestration), hepcidin (reducing serum iron), and mannose-binding lectin. Negative acute phase proteins, including albumin and transferrin, decrease as hepatic synthetic capacity is redirected.

Resolution of inflammation is an active process requiring specific molecular programs rather than simply passive decay of inflammatory signals. Pro-resolving lipid mediators, including lipoxins, resolvins, protectins, and maresins, are synthesized from omega-3 and omega-6 fatty acids and actively suppress neutrophil recruitment while promoting macrophage phagocytosis of apoptotic cells (efferocytosis). Anti-inflammatory cytokines, particularly IL-10 and TGF-β, suppress pro-inflammatory cytokine production and promote tissue repair. Regulatory T cells recruited to inflammatory sites produce these same cytokines. Failure of inflammation resolution leads to chronic inflammation with persistent tissue damage, as occurs in chronic inflammatory diseases like rheumatoid arthritis and inflammatory bowel disease.

<image> Panel A: Diagram showing cytokine-mediated fever induction, with IL-1β, IL-6, and TNF-α acting on hypothalamic endothelium to induce COX-2 and PGE2 synthesis, PGE2 acting on thermoregulatory neurons to raise the temperature set point, and effector responses (vasoconstriction, shivering) that increase body temperature Panel B: Table visualization of acute phase proteins showing positive acute phase reactants (CRP, ferritin, fibrinogen, hepcidin, serum amyloid A, complement components, MBL) with their functions and fold-increases, and negative acute phase reactants (albumin, transferrin) with their decreases Panel C: C-reactive protein mechanism illustration showing CRP binding to phosphocholine on bacterial membranes and apoptotic cell surfaces, with subsequent C1q binding and classical complement pathway activation, promoting phagocytosis through C3b opsonization Panel D: Resolution of inflammation diagram showing pro-resolving mediators (lipoxin A4, resolvin E1, protectin D1) inhibiting neutrophil recruitment and promoting macrophage efferocytosis of apoptotic neutrophils, with IL-10 and TGF-β suppressing cytokine production and promoting tissue repair </image>

X. Interface with Adaptive Immunity

The transition from innate to adaptive immunity depends critically on dendritic cells, which integrate information from peripheral tissues about pathogen encounter and translate this into signals that direct appropriate T cell responses. When dendritic cells capture antigen in tissues and receive activation signals through pattern recognition receptors, they undergo a maturation program that transforms them from phagocytic sentinels into potent antigen-presenting cells. This maturation involves migration to draining lymph nodes, upregulation of MHC class II molecules loaded with pathogen-derived peptides, expression of costimulatory molecules, and production of cytokines that shape T cell differentiation.

T cell activation requires three signals from dendritic cells, each essential for generating an effective adaptive response. Signal 1 comprises the antigen-specific interaction between the T cell receptor and peptide-MHC complex, providing specificity to the response. Signal 2 involves costimulatory molecule interactions, particularly CD28 on T cells binding CD80 and CD86 on dendritic cells. This costimulation prevents anergy induction and promotes T cell survival, proliferation, and effector function. Without signal 2, antigen recognition leads to T cell unresponsiveness, providing a mechanism for peripheral tolerance. Signal 3 comprises cytokines produced by dendritic cells that direct T helper cell differentiation: IL-12 promotes Th1 development, IL-4 promotes Th2, and IL-6 plus TGF-β promote Th17.

The nature of the innate immune activation profoundly influences the type of adaptive response generated. Intracellular pathogens triggering TLR3, TLR7/8, or TLR9 (nucleic acid sensors) induce IL-12 production, promoting Th1 responses and cellular immunity. Helminths and allergens promote Th2 responses through mechanisms involving IL-4 production, though the innate pathways leading to type 2 responses are less well characterized. Extracellular bacteria and fungi activating TLR2 and Dectin-1 promote IL-6 and IL-23 production, favoring Th17 responses. This integration ensures that the adaptive response is appropriate for the type of pathogen encountered.

Complement provides additional links between innate and adaptive immunity. C3d fragments covalently attached to antigens bind CR2 (CD21) on B cells, lowering the threshold for B cell activation up to 1000-fold. This explains why complement-opsonized antigens are highly immunogenic and why some complement deficiencies impair antibody responses. Follicular dendritic cells in lymph node germinal centers retain complement-coated immune complexes for extended periods, providing a depot of antigen for B cell affinity maturation. Additionally, complement split products affect T cell responses through complement receptors on antigen-presenting cells and T cells themselves, though these mechanisms are still being characterized.

<image> Panel A: Dendritic cell maturation and migration diagram showing immature DC in peripheral tissue capturing pathogen and receiving TLR signals, CCR7-directed migration through lymphatics, and arrival in T cell zone of lymph node as mature DC with high MHC II, CD80/86, and cytokine production Panel B: Three-signal model of T cell activation showing Signal 1 (TCR engaging peptide-MHC on DC), Signal 2 (CD28 engaging CD80/86), and Signal 3 (cytokines directing differentiation), with outcomes of each signal combination (activation, anergy, polarized response) Panel C: Innate instruction of adaptive immunity diagram showing different PRR pathways leading to specific cytokine profiles that direct T helper differentiation - TLR9/IL-12 to Th1, allergen-induced IL-4 to Th2, TLR2+Dectin-1/IL-6+IL-23 to Th17 Panel D: Complement-enhanced B cell activation showing antigen with attached C3d binding simultaneously to BCR (antigen recognition) and CR2/CD21 (complement receptor), with co-ligation dramatically lowering activation threshold, and follicular dendritic cell retention of complement-coated immune complexes for affinity maturation </image>

Summary

• Innate immunity provides immediate, non-specific defense without immunological memory, relying on germline-encoded receptors
• Physical and chemical barriers including skin, mucosal surfaces, and antimicrobial peptides prevent pathogen entry
• Pattern recognition receptors (TLRs, NLRs, RLRs, CLRs) detect PAMPs and DAMPs to initiate immune responses
• Neutrophils are first-responder phagocytes using oxidative burst and NETs; macrophages are tissue-resident with M1/M2 polarization
• NK cells kill infected and tumor cells through missing-self recognition, using perforin/granzyme and producing IFN-γ
• The complement system activates via classical, lectin, and alternative pathways leading to opsonization (C3b), inflammation (C3a, C5a), and lysis (MAC)
• Complement regulation prevents host damage; deficiencies cause infection susceptibility or autoimmunity
• Inflammation involves vascular changes, leukocyte recruitment, and cytokine signaling (TNF-α, IL-1β, IL-6)
• The acute phase response produces fever, leukocytosis, and acute phase proteins including CRP
• Dendritic cells bridge innate and adaptive immunity by providing three signals for T cell activation

Key Terms

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